Revert running tests in parallel

See gh-19876

buildSrc/src/main/java/org/springframework/boot/build/JavaConventions.java

@@ -156,7 +156,6 @@ class JavaConventions {
 		project.getTasks().withType(Test.class, (test) -> {
 			test.useJUnitPlatform();
 			test.setMaxHeapSize("1024M");
-			test.setMaxParallelForks(project.getGradle().getStartParameter().getMaxWorkerCount());
 			project.getTasks().withType(Checkstyle.class, (checkstyle) -> test.mustRunAfter(checkstyle));
 			project.getTasks().withType(CheckFormat.class, (checkFormat) -> test.mustRunAfter(checkFormat));
 		});